FIG. 1 illustrates a grid-connected photovoltaic system, which includes N (N≥1) photovoltaic components 100, N photovoltaic optimizers 200 (which may have a bust topology or a bi-directional DC-DC conversion topology) and one photovoltaic inverter 300. The N photovoltaic components 100 are respectively connected to input sides of the N photovoltaic optimizers 200, and output sides of the N photovoltaic optimizers 200 are connected to a DC side of the photovoltaic inverter 300 (a case where N photovoltaic components 100 form one photovoltaic string is shown in FIG. 1 only as an example). Each photovoltaic optimizer 200 is used for performing Maximum Power Point Tracking (MPPT) control on the photovoltaic component 100 connected thereto, such that a component-level MPPT function can be implemented in the gird-connected photovoltaic system.
However, since the photovoltaic inverter 300 also has the MPPT function, an overall conversion efficiency of the gird-connected photovoltaic system may be reduced when the photovoltaic optimizer 200 in a preceding stage and the photovoltaic converter 300 in a succeeding stage on each branch perform MPPT control independently.
One of the branches (shown by the dotted box in FIG. 1) is taken as an example. On this branch, the photovoltaic optimizer 200 independently tracks a maximum output power of the photovoltaic component 100, and the photovoltaic inverter 300 independently tracks a maximum input power of the DC side of the photovoltaic inverter 300. When the photovoltaic optimizer 200 acquires the maximum output power of the photovoltaic component 100, the higher the voltage of the DC side of the photovoltaic inverter 300, the greater the duty cycle of photovoltaic optimizer 200; the greater the duty cycle of photovoltaic optimizer 200, the higher the conversion efficiency of the photovoltaic optimizer 200; and the higher the conversion efficiency of the photovoltaic optimizer 200, the greater the input power of the DC side of the photovoltaic inverter 300. Therefore, as a result of the photovoltaic inverter 300 independently tracking the maximum input power of the DC side, the voltage of the DC side of the photovoltaic inverter 300 is maintained at the maximum value. However, when the voltage of the DC side of the photovoltaic inverter 300 reaches the maximum value, the photovoltaic inverter 300 has the lowest conversion efficiency. Therefore, the overall conversion efficiency of the branch is not optimal, and even can be very low. Therefore, the overall conversion efficiency of the grid-connected photovoltaic system is low.